[fix: gdb parser - callstack]
[fix: gdb parser - callstack]

	issue: frame-list (callstack) with inverted order of frames caused
		   creation of unnecessary variables and wrong 'callstack'
		   window ordering
	fix: used list_add_head() instead of list_add_tail() when parsing a
		 callstack

jnow-87/vimgdb,jnow-87/vimgdb,jnow-87/vimgdb