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
add talloc0 function (malloc and zero fill)
check return values of arithmetic openssl BN_* functions
check return values of BN_bin2bn function
remove useless BN_init calls right after BN_new
fix 2 small memory leaks