reallocarr(3) cleanup

["Kamil Rytarowski" <n54%gmx.com@localhost>]

Hello,

I'm attaching a patch with reallocarr{.ay}(3) cleanup:
- merge reallocarr.3 to malloc.3 for consistency,
- make the reallocarr(3) documentation more detailed,
- write history in malloc.3 of function allocations,
- add in malloc.3 warning about possible overflows,
- add erallocarr(3) to libutil,
- add missing Id tag to reallocarray.c,
- sync reallocarr prototype with documentation (num -> number),
- note that reallocarr first appeared in .Nx 8 not 7 (am I right about it?).

Please review the English correctness.

src/lib/libc/stdlib/reallocarray.3 is without licensing notes! please add it.

CC: authors of the new function.

Thank you for your time!