ShellPkg/help: Fix "-?" may not show manual sometimes

ShellPkg/help: Fix "-?" may not show manual sometimes
Shell core was enhanced to find the manual string in PE resource
section. But the finding algorithm is too strict: If the manual is
written beginning with:
.TH command 0 "descripton of command"

but user types "COMMAND.efi -?". The finding algorithm uses
case-sensitive compare between "command" and "COMMAND" resulting
in the manual cannot be found.

The patch fixes this issue by using existing ManFileFindTitleSection
and ManFileFindSections which compare command case-insensitive.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>