Make error messages about WAL segment size more consistent

This started out as a small patch to make pg_controldata use the logging 
API instead of printf statements, and then it became a larger patch to 
adjust error and warning messages about invalid WAL segment sizes 
(IsValidWalSegSize()) across the board.  I went through and made the 
primary messages more compact and made the detail messages uniform.  In 
initdb.c and pg_resetwal.c, I use the newish option_parse_int() to 
simplify some of the option parsing.  For the backend GUC 
wal_segment_size, I added a GUC check hook to do the verification 
instead of coding it in bootstrap.c.  This might be overkill, but that 
way the check is in the right place and it becomes more self-documenting.