Convert $header_cache_pagesize to type DT_LNUM.

Convert $header_cache_pagesize to type DT_LNUM.

Prior to commit 4bc76c2f there was no LNUM type, and so the workaround
was to store it as a string, converting in the hcache_open_gdbm()
call.

This will not affect the user interface or config file, because DT_NUM
and DT_LNUM read in a string from the config file and convert to a
number.  Quotes are used for escaping style, not passed through to the
variable setter.

So essentially this simply moves the conversion to parse_set(), and
provides feedback for a non-numeric type immediately.