Update interface.c to avoid error messages

When hitting enter (without writing anything) in an chat prompt always appears an error message telling you that the message is empty.
Checking the content of the line can avoid this.
I don't know is this behaviour is wrong or right.
So please forgive me if it doesn’t make sense.

Complete translation and small typing errors corrected

Update README.md and create README.es

Fedora don't use commas in his installation system to separate packages

Update README with new automatic path for server.pub

Help text updated.

I just added "You can see message numbers starting client with -N" to the load_photo and the view_photo description.
I've been in trouble because of not knowing how to download files so I think its use-full.
I'm not sure if it's how github works my idea was to Propose this change to the main branch.

Updated rpm with new spec

Basic Debian/Ubuntu package

Adding initial spec to build from source

minor configure.ac simplifications

please add AC_CHECK_LIB([ssl], [function-name]) to configure.ac in the case of failed linkage

fix unused write result in undefined HAVE_EXECINFO_H case

remove printf from signal handlers and reduce configure.ac

use AC_SEARCH_LIB in configure.ac (shorter code)
replace '-lm -lcrypto -lz' by  @LIBS@ in Makefile.in and AC_CHECK_LIB macros in configure.ac
replace NO_BACKTRACE by HAVE_EXECINFO_H