Integrate changes #11193, 11205, 11209 from macperl.

Integrate changes #11193, 11205, 11209 from macperl.

Fix up AutoLoader to fudge for Mac paths in import().

Allow symbols in try_symbol() to begin with "_".

Small portability fix for Mac OS.

p4raw-link: @11209 on //depot/maint-5.6/macperl: 6ad6b2be750c4cb30e974afa9b2e179a2f381679
p4raw-link: @11205 on //depot/maint-5.6/macperl: 85661f92e975162c5b6280ce9e0b3cd6ee7563fd
p4raw-link: @11193 on //depot/maint-5.6/macperl: 87cd12a80943bf11d0d777d1855d8572168f9887

p4raw-id: //depot/perl@11219
p4raw-integrated: from //depot/maint-5.6/macperl@11217 'merge in'
	lib/AutoLoader.pm lib/Cwd.pm makedef.pl (@11007..)