potx 5.x-1.3

Changes since 5.x-1.2:

• #230413 by suppasonic: fix exporting plural translations
• #298996 reported gusgsm1, patch by myself, backported: gracefully handle cases when glob() does not return with data and then consequently there are possibly no strings in the global store
• #263790 reported by Micha1111, patch by myself: turns out that GLOB_BRACE is not available on all operating systems, we need to have our own workaround code or it bails miserably
• #288117 reported by Bartezz, fixed by myself: avoid that people run potx-cli.php from the web, since it was not designed so. tell them to use the module web interface.
• Backporting #192773
• #219007 report and patch by hass: small installer files were merged into general.po, creating duplicate strings; they should not be merged (only applicable to 5.x versions of potx)
• #204981 by phirdina: add missing paragraph to help output