use reentrant getmntent_r

This fixes the following data-race on VLC when 2 different inputs are opening
at the same time:

WARNING: ThreadSanitizer: data race (pid=15659)
  Write of size 8 at 0x7f79148d3140 by thread T5:
    #0 getmntent ../../../../src/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:3850 (libtsan.so.0+0x3d84c)
    #1 DVDOpenCommon ../src/dvd_reader.c:548 (libdvdread.so.4+0x38b6)
    #2 generic_start ../../src/modules/modules.c:258 (libvlccore.so.9+0x3bed5)
    #3 module_load ../../src/modules/modules.c:122 (libvlccore.so.9+0x3be7e)
    #4 vlc_module_load ../../src/modules/modules.c:194 (libvlccore.so.9+0x3c358)
    #5 module_need ../../src/modules/modules.c:277 (libvlccore.so.9+0x3c60c)
    #6 access_New ../../src/input/access.c:149 (libvlccore.so.9+0x5464c)
    #7 stream_AccessNew ../../src/input/access.c:294 (libvlccore.so.9+0x549c1)
    #8 InputDemuxNew ../../src/input/input.c:2413 (libvlccore.so.9+0x7528b)
    #9 InputSourceNew ../../src/input/input.c:2556 (libvlccore.so.9+0x76c1c)
    #10 Init ../../src/input/input.c:1257 (libvlccore.so.9+0x7b145)
    #11 Run ../../src/input/input.c:449 (libvlccore.so.9+0x7c3fe)

  Previous write of size 8 at 0x7f79148d3140 by thread T4:
    #0 getmntent ../../../../src/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:3850 (libtsan.so.0+0x3d84c)
    #1 DVDOpenCommon ../src/dvd_reader.c:548 (libdvdread.so.4+0x38b6)
    #2 generic_start ../../src/modules/modules.c:258 (libvlccore.so.9+0x3bed5)
    #3 module_load ../../src/modules/modules.c:122 (libvlccore.so.9+0x3be7e)
    #4 vlc_module_load ../../src/modules/modules.c:194 (libvlccore.so.9+0x3c358)
    #5 module_need ../../src/modules/modules.c:277 (libvlccore.so.9+0x3c60c)
    #6 access_New ../../src/input/access.c:149 (libvlccore.so.9+0x5464c)
    #7 stream_AccessNew ../../src/input/access.c:294 (libvlccore.so.9+0x549c1)
    #8 InputDemuxNew ../../src/input/input.c:2413 (libvlccore.so.9+0x7528b)
    #9 InputSourceNew ../../src/input/input.c:2556 (libvlccore.so.9+0x76c1c)
    #10 Init ../../src/input/input.c:1257 (libvlccore.so.9+0x7b145)
    #11 Preparse ../../src/input/input.c:475 (libvlccore.so.9+0x7c2b6)