Make memset in calloc a tail call.
Get canonical name in getaddrinfo from hosts file for AF_INET (fixes 16077)

AF_INET lookup in hosts file uses _nss_files_gethostbyname2_r, which
is not capable of returning a canonical name if it has found one.
This change adds _nss_files_gethostbyname3_r, which wraps around
_nss_files_gethostbyname2_r and then returns result.h_name as the
canonical name.
Fix dbl-64 e_sqrt.c for non-default rounding modes (bug 16271).
Document some libm error handling intents.
Add powerpc-nofpu/e500 support functions for atomic compound assignment and FLT_ROUNDS.
Remove unused ldbl-96 functions (bug 15004).
Document libm accuracy goals.