common/getpart: added a function to properly parse headers.

Mail <FC.10.09355.8BF89F05@pb1.pair.com> on php.internals included a patchfile with a very long name which the newsserver/the mailprogram split into two parts:
 filename*0="0001-Improve-x86-assembler-implementations-of-add-sub-int.pa";
 filename*1="tch"

The new function reassembles the split headers and allows proper browser output.