Improve case-insensitive checks (Bug#24441)

Improve case-insensitive checks (Bug#24441)

* doc/lispref/files.texi (Truenames): Simplify documentation,
to avoid giving too much platform-specific information that
may not be accurate anyway.
* src/fileio.c (file_name_case_insensitive_p): Use pathconf with
_PC_CASE_SENSITIVE if _PC_CASE_INSENSITIVE is not available.
Otherwise if one approach fails (e.g., with errno == EINVAL), fall
back on an alternative rather than returning false.  Try skipping
the Darwin code, as it (1) no longer seems to be needed and (2)
does not seem to match the Apple documentation.  Leave in two
alternatives conditionally compiled based on
DARWIN_OS_CASE_SENSITIVE_FIXME in case (1) or (2) is incorrect.