* Add config switch to disable (x)proofd. Default OFF.

This patch disables default building proofd and proofx modules, the PROOF
ones requiring XRootD. The possibility to build these modules is provided
as legacy and controlled by the switch 'xproofd' (-Dxproofd=ON).
Note that these modules require XRootD v4 and the xrootd-private-devel
RPM installed (or equivalent on Debian systems). Or -Dbuiltin_xrootd=ON .

* Proof: fix segv introduced by patch 7a810c6f (2020-02-14)

These executable do not linked directly to Core lib,
therefore required to specify include paths directly

Otherwise one has to set dependency from ProofPlayer lib

It is a proper place for virtual class

TProofServ::GetEnabledPackages() creates TList, which should be deleted
with content. TProof::GetEnabledPackages() does not create new variable
and therefore consumer should not delete it. But does not cleanup in
destructor

TList and contained objects must be deleted

Returned TList object should be deleted

Whichever library was missing a link against libatomic in my
Raspberry Pi test build, received a link against ROOT_ATOMIC_LIBS
in this update.

V595 The 'fPlayer' pointer was utilized before it was verified against nullptr. Check lines: 3425, 3430. TProof.cxx 3425
V595 The 'gProofServ' pointer was utilized before it was verified against nullptr. Check lines: 1192, 1194. TProofPlayer.cxx 1192

V593 Consider reviewing the expression of the 'A = B < C' kind. The expression is calculated as following: 'A = (B < C)'. TProofServ.cxx 1903
V593 Consider reviewing the expression of the 'A = B < C' kind. The expression is calculated as following: 'A = (B < C)'. TProofServ.cxx 3897

This bug reveals itself only in the case of the program's faulty behavior. The ret variable is supposed to store the return code of the AddWorkers function and write that value to the log in case of error condition. But it doesn't work as intended. The condition lacks additional parentheses forcing the desired order of evaluation. What the ret variable actually stores is not the return code but the result of the logical comparison, i.e. either 0 or 1.

V517 The use of 'if (A) {...} else if (A) {...}' pattern was detected. There is a probability of logical error presence. Check lines: 163, 165. TProofMonSenderML.cxx 163

V646 Consider inspecting the application's logic. It's possible that 'else' keyword is missing. TProofServ.cxx 729
V646 Consider inspecting the application's logic. It's possible that 'else' keyword is missing. TFormula_v5.cxx 3702
V646 Consider inspecting the application's logic. It's possible that 'else' keyword is missing. RooAbsCategory.cxx 604

Related issues: ROOT-10127, ROOT-10138

fPool should be used in Form() when it's not null,
not the reverse.

Related issues: ROOT-10127, ROOT-10138

Can cause unnecessary extra linking to system libraries. We should
add linking libraries directly to each target that needs it.