Fix a few typos in comments

Fix a few typos in comments

Apply the following spelling fixes:
$ git grep -F -l 'relevent' |
  xargs sed -i 's/relevent/relevant/g'
$ git grep -F -l 'checked fot' |
  xargs sed -i 's/checked fot/checked for/g'
$ git grep -F -l "could't" |
  xargs sed -i "s/could't/couldn't/g"
$ git grep -F -l 'wheter' | grep -Fv ChangeLog.old |
  xargs sed -i 's/wheter/whether/g'
$ git grep -F -l 'neccessary' | grep -Fv ChangeLog.old |
  xargs sed -i 's/neccessary/necessary/g'
$ git grep -F -l 'ouput' |
  xargs sed -i 's/ouput/output/g'
$ git grep -F -w -l 'iput' |
  xargs sed -i 's/iput/input/g'

This is inspired by a gnulib bug report at
https://lists.gnu.org/archive/html/bug-gnulib/2019-01/msg00081.html

* argp/argp-help.c: Fix typo in comment.
* misc/sys/cdefs.h: Likewise.
* posix/regexec.c (sift_states_iter_mb): Likewise.
* socket/sockatmark.c: Likewise.
* socket/sys/socket.h: Likewise.
* sysdeps/ia64/fpu/libm_sincos_large.S: Likewise.
* sysdeps/ia64/fpu/libm_sincosl.S: Likewise.
* sysdeps/ia64/fpu/s_cosl.S: Likewise.
* sysdeps/ieee754/dbl-64/k_rem_pio2.c: Likewise.
* sysdeps/unix/sockatmark.c: Likewise.
* time/strptime_l.c: Likewise.