* environ.cc: Add extern declaration for `pcheck_case'.
(check_case_init): New function.
(struct parse_thing): Add "check_case" option.
* errno.cc (_sys_nerrlist): Add text for ECASECLASH.
(strerror): Add case branch for ECASECLASH.
* fhandler.cc (fhandler_disk_file::open): Check for case clash.
* path.cc: Add global variable `pcheck_case'.
(struct symlink_info): Add member `case_clash' and method `case_check'.
(path_prefix_p_): Call `pathnmatch' instead of `strncasematch'.
(pathnmatch): New funtion.
(pathmatch): Ditto.
(path_conv::check): Add handling for case checking.
(symlink): Check for case clash.
(symlink_info::check): Add parameter for case checking.
Handle case checking.
(symlink_info::case_check): New method.
(chdir): Don't use unconverted path if pcheck_case==PCHECK_STRICT.
* path.h: Add extern declarations for `pathmatch' and
`pathnmatch'.
(enum case_checking): New enumeration type describing
the case checking behaviour of path conversion routines.
(class path_conv): Add member `case_clash'.
* syscalls.cc (_link): Check for case clash.
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