Avoid gcc 9.1 strncpy(3) warnings (#447)

Avoid gcc 9.1 strncpy(3) warnings (#447)

Recent GCC releases have warnings related to common strncpy(3) bugs.
These warnings can be avoided by explicitly NUL-terminating the buffer
or using memcpy(3) when the intention is to copy just the characters
without the NUL terminator.

This commit fixes the following warnings:

  [1/27] Compiling C object 'test/9f86d08@@test_syscalls@exe/test_syscalls.c.o'.
  In function ‘test_socket’,
      inlined from ‘main’ at ../test/test_syscalls.c:1899:9:
  ../test/test_syscalls.c:1760:2: warning: ‘strncpy’ output may be truncated copying 108 bytes from a string of length 1023 [-Wstringop-truncation]
   1760 |  strncpy(su.sun_path, testsock, sizeof(su.sun_path));
        |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  [2/27] Compiling C object 'lib/76b5a35@@fuse3@sha/fuse.c.o'.
  ../lib/fuse.c: In function ‘add_name’:
  ../lib/fuse.c:968:2: warning: ‘strncpy’ output truncated before terminating nul copying as many bytes from a string as its length [-Wstringop-truncation]
    968 |  strncpy(s, name, len);
        |  ^~~~~~~~~~~~~~~~~~~~~
  ../lib/fuse.c:944:15: note: length computed here
    944 |  size_t len = strlen(name);
        |               ^~~~~~~~~~~~
  [3/27] Compiling C object 'lib/76b5a35@@fuse3@sha/fuse_lowlevel.c.o'.
  ../lib/fuse_lowlevel.c: In function ‘fuse_add_direntry’:
  ../lib/fuse_lowlevel.c:288:2: warning: ‘strncpy’ output truncated before terminating nul copying as many bytes from a string as its length [-Wstringop-truncation]
    288 |  strncpy(dirent->name, name, namelen);
        |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  ../lib/fuse_lowlevel.c:276:12: note: length computed here
    276 |  namelen = strlen(name);
        |            ^~~~~~~~~~~~
  ../lib/fuse_lowlevel.c: In function ‘fuse_add_direntry_plus’:
  ../lib/fuse_lowlevel.c:381:2: warning: ‘strncpy’ output truncated before terminating nul copying as many bytes from a string as its length [-Wstringop-truncation]
    381 |  strncpy(dirent->name, name, namelen);
        |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  ../lib/fuse_lowlevel.c:366:12: note: length computed here
    366 |  namelen = strlen(name);
        |            ^~~~~~~~~~~~

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>