Fix some prototypes in manpage synopses.
Reduce diffs between GENERIC/X86_64_GENERIC a bit.
kernel: Adjust some header comments. SMP is default now for a while.
porting_drivers.txt: Reword the part about SVN IDs a bit.
share/mk: Fix the creation of MLINKS for manpages of fschg'd stuff.

We cannot pass INSTALLFLAGS directly for manual page installation because
when INSTALLFLAGS contains -fschg (such as for init(8)), it will cause the
manual page to be fschg'd too which breaks subsequent MLINKS creation.

Therefore, use MANINSTALLFLAGS for manual pages. While here, document it
too.
GC a manpage backup that was created by passing -b when installing it.