Sync procmime.c with gtk2 branch, use my_tmpfile instead of old scheme:

Sync procmime.c with gtk2 branch, use my_tmpfile instead of old scheme:
this, in addition to previous commit (ce4baa225a17648a19c3806dac4f57cbd938b3ef), is mostly
a backport the sequence of 414b08d658d56f25052aab18ab75d7e321d97e3b, cb3f6bfba779f53100a9b21c09cb9489ad7469d0
and 77f9a0f030366ec10579134c255ef1781c1e55ab from master (when master was gtk2) that was only half-merged
to the gtk3 branc.