Assorted build fixes and improvements

This includes the fix from !2029 (closed) among other minor fixes.

• build: ignore new functions at "make abi-check-latest"

• build: pacify GCC analyzer false-positive in src/ocsptool.c

Without the guard (chain_size - 1), GCC analyzer spews the warning below, which should be a false-positive:

ocsptool.c:532:32: warning: use of uninitialized value 'chain[1]' [CWE-457] [-Wanalyzer-use-of-uninitialized-value]
  532 |                         signer = chain[1];
      |                         ~~~~~~~^~~~~~~~~~

• lib: Fix Wunterminated-string-initialization warnings

Building on a newer gcc version (15) results in the following warnings:

status_request.c: In function 'client_send':
status_request.c:71:33: warning: initializer-string for array of 'unsigned char' truncates NUL terminator but destination lacks 'nonstring' attribute (6 chars into 5 available) [-Wunterminated-string-initialization]
   71 |         const uint8_t data[5] = "\x01\x00\x00\x00\x00";
      |                                 ^~~~~~~~~~~~~~~~~~~~~~
x86-common.c: In function 'check_phe_partial':
x86-common.c:342:31: warning: initializer-string for array of 'char' truncates NUL terminator but destination lacks 'nonstring' attribute (65 chars into 64 available) [-Wunterminated-string-initialization]
  342 |         const char text[64] = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
      |
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

• build: exclude duplicate entries in src/mech-list.h

• build: derive the location of default config file from $sysconfdir

Previously we hard-coded "/etc" as part of the path of the default configuration file. It is more palatable to respect the --sysconfdir configure option and locate the file there.

Per recommendation at [1], the path is expanded at "make" time, not at "configure" time.

1. https://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.72/html_node/Installation-Directory-Variables.html

Checklist

• Commits have Signed-off-by: with name/author being identical to the commit author
• Code modified for feature
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated / NEWS entry present (for non-trivial changes)

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTION.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code