Don't assume that int is signed 32-bits in one place. Keep assuming

Don't assume that int is signed 32-bits in one place.  Keep assuming
that ints have >= 31 value bits elsewhere.  s/int/int32_t/ seems to
have been done too globally for all other files in msun/src before
msun/ was imported into FreeBSD.

Minor fixes in comments.

e_lgamma_r.c:
Describe special cases in more detail:
- exception for lgamma(0) and lgamma(neg.integer)
- lgamma(-Inf) = Inf.  This is wrong but is required by C99 Annex F.  I
  hope to change this.