Draft: Merge branch Mario/misc_tweaks (M merge request)

Several tweaks centered around fixing some of the code errors that compiling with FTEQCC exposes, along with partial support for FTEQCC as a compiler option.

Noteworthy changes:

• One can now customise the previously hidden QCCIDENT build flag to switch the gamecode from GMQCC to FTEQCC supported extensions during build
• The QC compiler's targeted extensions can now also be customised via QCCFLAGS_TARGET - though replacements are currently rather buggy (FTEQCC's -Tdp flag causes crashes on launch)
• References to ** (power exponent) have been wrapped in a pow macro to allow compilers to define their own support (FTEQCC uses *^)
• Net_LinkEntity has been given a new parameter to disable checking whether the entity has a valid model so that it may be used in cases where the SendEntity engine field was directly referenced, and those cases have been switched to use it
• Any functions that make use of accumulation (a QC compiler extension) are now explicitly accumulated, for compatibility with FTEQCC
• The only reference to a legacy engine function checkclient has been removed - it did nothing in the odd cases where it was called (projectiles colliding with eachother)