Change license for some files in common to LGPLv3+/GPLv2+.

Having the LGPL on the common GnuPG code helps to share code
between GnuPG and related projects (like GPGME and Libassuan). This
is good for interoperability and to reduces bugs.

• common/asshelp.c, common/asshelp.h, common/asshelp2.c, common/b64dec.c
• common/b64enc.c, common/convert.c, common/dns-cert.c
• common/dns-cert.h common/exechelp-posix.c, common/exechelp-w32.c
• common/exechelp-w32ce.c, common/exechelp.h, common/get-passphrase.c
• common/get-passphrase.h, common/gettime.c, common/gpgrlhelp.c
• common/helpfile.c, common/homedir.c, common/http.c, common/http.h
• common/i18n.c, common/init.c, common/init.h, common/iobuf.c
• common/iobuf.h, common/localename.c, common/membuf.c, common/membuf.h
• common/miscellaneous.c, common/openpgp-oid.c, common/openpgpdefs.h
• common/percent.c, common/pka.c, common/pka.h, common/session-env.c
• common/session-env.h, common/sexp-parse.h, common/sexputil.c
• common/signal.c, common/srv.c, common/srv.h, common/ssh-utils.c
• common/ssh-utils.h, common/sysutils.c, common/sysutils.h
• common/tlv.c, common/tlv.h, common/ttyio.c, common/ttyio.h
• common/userids.c, common/userids.h, common/xasprintf.c: Change

license to LGPLv3+/GPLv2+/