Add guard code to protect from buffer overruns on long date/time input

Add guard code to protect from buffer overruns on long date/time input
 strings. Should go back in and look at doing this a bit more elegantly
 and (hopefully) cheaper. Probably not too bad anyway, but it seems a
 shame to scan the strings twice: once for length for this buffer overrun
 protection, and once to parse the line.
Remove use of pow() in date/time handling; was already gone from everything
 *but* the time data types.
Define macros for handling typmod manipulation for date/time types.
 Should be more robust than all of that brute-force inline code.
Rename macros for masking and typmod manipulation to put TIMESTAMP_
 or INTERVAL_ in front of the macro name, to reduce the possibility
 of name space collisions.