Fix FONTFORGE_ARG_WITH_CAIRO() to hard-stop if "--with" but no cairo

Some operating systems have a weaker pkg-config, and won't find CAIRO if
looking only for a package. Therefore, try look for the package first,
and then if failed, try looking for library and header files.
Check for silly options like trying to specify with Cairo, but no png.
minor alphabetization of options. Remove some dead code for *.pc
Improve spline overlap handling.
Attempt to fix handling of quadratic splines with no on-curve points.
Wait to set the leading control point until we're making the curve.
Handle an extensionless file that opens without crashing in LoadSplineFont.
Merge pull request #1518 from fontforge/overlap_5

Improve spline overlap handling.
Merge pull request #1527 from fontforge/issue_1517_1

Attempt to fix handling of quadratic splines with no on-curve points.
Disable code that aborts switching to preview mode.
Merge pull request #1529 from fontforge/issue_1528_exploration

Handle an extant extensionless file without crashing in LoadSplineFont.
po: fix a teeny issue in French translation
Merge pull request #1531 from adrientetar/more-fr

po: fix a teeny issue in French translation
mmdlg: fix a memory leak
showatt: fix a potential memory leak
splinefont: fix some memory leaks
Drop leftovers of broken-size output for TTF files

(Note that support for reading these early broken files is still
there, in parsettfatt.c – which is good.)
Drop (ancient) OFLib support
Merge pull request #1530 from fontforge/issue_1514_exploration

Disable code that aborts switching to preview mode.
Fix MIME detection by adding missing comma
Frank wants NULL ptrs