This avoids creating of incorrect symlink ${HOME}/sys -> usr/src/sys when
DESTDIR="".

All POSIX libc api now there (to P1003.1c/D10)
(more md stuff is needed for other libc/arch/*)
(setlogin is no longer a special syscall)
Add -pthread option to gcc (that makes it use -lc_r and -D_POSIX_THREADS).
Doc some re-entrant routines
Add libc_r to intro(3)
dig() uses some libc srcs and an extra -I was needed there.
Add more md stuff to libc_r.
Update includes for the pthreads api
Update libc_r TODO

in ppp.linkdown.

a function, isalphanumeric, to determine whether an interface name is
likely to be valid.  This means that things like /etc/hostname.le0.bak,
/etc/hostname.le0#, /etc/hostname.le0~, etc. will be ignored as they
should.  There is no longer an implicate assumption that /etc/hostname.*
only contains a single '.'.