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The configure script relied on bash-specific extensions to shell syntax
and behavior, causing build failures on systems with other /bin/sh
implementations. This commit replaces those with equivalent constructs
that should work in all POSIX shells.

Fixes #695.

glibc 2.28 no longer defines 'major' and 'minor' in <sys/types.h> and
requires us to include <sys/sysmacros.h>. (glibc 2.25 starts
deprecating the macros in <sys/types.h>.) Now do include the latter if
found on the system.

At the moment, let's also utilize AC_HEADER_MAJOR in configure script.
However as Autoconf 2.69 has not yet updated the AC_HEADER_MAJOR macro
to reflect the glibc change [1], so add a workaround code.

Fixes #663. Supersedes pull request #729.

Reference:
[1] https://git.savannah.gnu.org/gitweb/?p=autoconf.git;a=commit;h=e17a30e987d7ee695fb4294a82d987ec3dc9b974

Signed-off-by: Kang-Che Sung <explorer09@gmail.com>

Avoid unnecessary access() call and make code read more linearly.

Require at least the `fields` entry to be present,
so we can have a decent guess that it was indeed a settings file.

I was occasionally passing negative values to size_t.
Plus, this better reflects the intent of the variant argument.

Reported by Coverity:
https://scan8.coverity.com/reports.htm#v13253/p10402/fileInstanceId=22093891&defectInstanceId=7543346&mergedDefectId=174179&fileStart=251&fileEnd=500

Detected by Coverity:
https://scan8.coverity.com/reports.htm#v13252/p10402/fileInstanceId=22093957&defectInstanceId=7543348&mergedDefectId=174180

Detected by Coverity:
https://scan8.coverity.com/reports.htm#v13252/p10402/fileInstanceId=22093847&defectInstanceId=7543344&mergedDefectId=174181

The "if" tests if the character at index "5" is 'r', as a first quick
check. However at index "5" will always be a colon ":". This patch fixes
the off-by-one error. htop now shows proper values in the RD_SYSC
column.
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

Fixes #738.