[GH #268] sysctl: check for HW_MEMSIZE

HW_PHYSMEM returns max 2G as its a signed int only. Use HW_MEMSIZE if available.
This helps rakudo building core.pir doing less gc, which is triggered when 10% of total memory is allocated
Merge remote-tracking branch 'origin/rurban/darwin-memsize-gh268'
[configure] quote libdir path for inst_libparrot_linkflags

otherwise pbc_to_exe does not work with spacey paths when installed
[doc] Update changelog
[doc][ci skip] Fix typo
Prepare for the 5.1.0 release
Add "Build" category to the changelog skeleton
Update version numbers for the 5.1.0 release
Update info for this release and next release
eliminate Tcl/Glob.pir
update manifest
[GH #933] Fix wrong blib rpath in installable_pdump

Thanks to he32++ for detecting this
[io] only sync buffers for IO types where it makes sense
add documentation on the new "all" dir on the ftp site