Parse key value pairs sent in POST body (#4627)

Backported from the master branch.

Conflicts:
	msautotest
fixups to build/test scripts
Merge branch 'branch-6-2'

Conflicts:
	msautotest
take into account style min/max scale in legend (#4601)

the mapscript and template code calling these updated functions
are hardcoded to do scale-independant legend rendering to remain
backwards compatible.
Fix segfault in readPostBody when CONTENT_LENGTH environmenet variable is not defined
mapwfs.c: correctly pair CPL allocation routines with CPL free routines
OGR WFS output: do not force null integer values to 0 (#4633)
update autotest submodule for #4633
Fix for WMS requests with width and height of 1 (#4629)
OGR WFS output: do not force null integer values to 0 (#4633)
update submodule for backported fix of #4633
updated submodule for #4633
Merge branch 'branch-6-2'

Conflicts:
	msautotest
CMAKE: Add zlib to the libraries list when png is added
Merge branch 'master' of https://github.com/mapserver/mapserver
update submodule after #4633 and #4499