patch 9.1.0731: inconsistent case sensitive extension matching

patch 9.1.0731: inconsistent case sensitive extension matching

Problem:  inconsistent case sensitive extension matching
Solution: unify case sensitive extension matching (Evgeni Chasnovski).

There are different approaches of how extensions are matched with
respect to case sensitivity. In particular, '\c' flag is used in pattern
whereas in most places case sensitive matching is guarded behind
`has("fname_case")` condition.

Replace all instances of '\c' with an explicit case sensitive pattern
variants guarded by `has("fname_case")`. Strictly speaking, this is a
breaking change because only two (most common and prevailingly tested)
variants are now matched: upper first letter and upper all letters.

closes: #15672

Signed-off-by: Evgeni Chasnovski <evgeni.chasnovski@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>