clang-format: adjust to sort C++ headers per style(9)

Many standard C++ headers do not have the .h file extension: some, such as
`iostream`, lack it; others have a .hpp file extension. Moreover, some
projects (like ATF/Kyua) also contain .ipp files, which are C++ "interface"
files.

Relax the regular expression to ensure that non-traditional C "system"
headers, C++ headers, etc, with angle brackets are sorted before
"local" headers.

MFC after:	1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D54401

.clang-format

@@ -151,10 +151,10 @@ IncludeCategories:
   - Regex: '^<(fs|nfs(|client|server)|ufs)/'
     Priority: 8
     SortPriority: 80
-  - Regex: '^<[^/].*\.h'
+  - Regex: '^<[^/].*'
     Priority: 9
     SortPriority: 90
-  - Regex: '^\".*\.h\"'
+  - Regex: '^\".*\"'
     Priority: 10
     SortPriority: 100
 # LLVM's header include ordering style is almost the exact opposite of ours.