gpg: simplify Python entry point and refactor Bash scripts a bit

Now there is a single 'trezor-gpg' tool, with various subcommands.

scripts/gpg-agent

@@ -0,0 +1,2 @@
+#!/bin/bash
+trezor-gpg agent

scripts/gpg-init

@@ -14,7 +14,7 @@ mkdir -p "${HOMEDIR}"
 chmod 700 "${HOMEDIR}"
 
 # Generate new GPG identity and import into GPG keyring
-trezor-gpg-create -v "${USER_ID}" -t "${TIMESTAMP}" -e "${CURVE}" > "${HOMEDIR}/pubkey.asc"
+trezor-gpg create -v "${USER_ID}" -t "${TIMESTAMP}" -e "${CURVE}" > "${HOMEDIR}/pubkey.asc"
 gpg2 --homedir "${HOMEDIR}" --import < "${HOMEDIR}/pubkey.asc"
 rm -f "${HOMEDIR}/S.gpg-agent"  # (otherwise, our agent won't be started automatically)
 
@@ -24,7 +24,7 @@ echo "${FINGERPRINT}:6" | gpg2 --homedir "${HOMEDIR}" --import-ownertrust
 
 # Prepare GPG configuration file
 echo "# TREZOR-based GPG configuration
-agent-program $(which trezor-gpg-agent)
+agent-program $(dirname ${0})/gpg-agent
 personal-digest-preferences SHA512
 " | tee "${HOMEDIR}/gpg.conf"
 

scripts/gpg-shell

@@ -13,10 +13,10 @@ then
 fi
 
 # Make sure that the device is unlocked before starting the shell
-trezor-gpg-unlock
+trezor-gpg unlock
 
 # Make sure TREZOR-based gpg-agent is running
-gpg-connect-agent --agent-program "$(which trezor-gpg-agent)" </dev/null
+gpg-connect-agent --agent-program "$(dirname $0)/gpg-agent" </dev/null
 
 COMMAND=$*
 if [ -z "${COMMAND}" ]