sh support

[sloanlance]

See the sh-support branch for code from user haochenx that adds support for shell I/O. Also see: https://stackoverflow.com/a/53273617/543738

Needs documentation and tests before opening a PR to upstream.

The qsh commands are meant to be "quiet" versions while the sh ones are verbose. I would change those around. sh should be quiet. qsh might be confused for a shell called "qsh", so they should be renamed to sh_verbose or just add a verbose flag to the sh commands.

The four blocks of code in src/builtin.c are very repetitive and need to be reduced. See:

jq/src/builtin.c

Lines 1565 to 1603 in 21da66b

	static jv f_sh_json(jq_state *jq, jv input, jv cmd) {
	jv_free(input);
	char *cmdstr = jq_sh_extract_cmdstr(cmd);
	if ((cmdstr) == NULL) return ret_error(cmd, jv_string(JQ_SH_EXTRACT_CMDSTR_ERRMSG));
	jv r = jq_sh(cmdstr, 0, 1);
	jv_mem_free(cmdstr);
	jv_free(cmd);
	return r;
	}
	static jv f_sh_str(jq_state *jq, jv input, jv cmd) {
	jv_free(input);
	char *cmdstr = jq_sh_extract_cmdstr(cmd);
	if ((cmdstr) == NULL) return ret_error(cmd, jv_string(JQ_SH_EXTRACT_CMDSTR_ERRMSG));
	jv r = jq_sh(cmdstr, 1, 1);
	jv_mem_free(cmdstr);
	jv_free(cmd);
	return r;
	}
	static jv f_qsh_json(jq_state *jq, jv input, jv cmd) {
	jv_free(input);
	char *cmdstr = jq_sh_extract_cmdstr(cmd);
	if ((cmdstr) == NULL) return ret_error(cmd, jv_string(JQ_SH_EXTRACT_CMDSTR_ERRMSG));
	jv r = jq_sh(cmdstr, 0, 0);
	jv_mem_free(cmdstr);
	jv_free(cmd);
	return r;
	}
	static jv f_qsh_str(jq_state *jq, jv input, jv cmd) {
	jv_free(input);
	char *cmdstr = jq_sh_extract_cmdstr(cmd);
	if ((cmdstr) == NULL) return ret_error(cmd, jv_string(JQ_SH_EXTRACT_CMDSTR_ERRMSG));
	jv r = jq_sh(cmdstr, 1, 0);
	jv_mem_free(cmdstr);
	jv_free(cmd);
	return r;
	}

Also see upstream issues stedolan#147 and stedolan#1101.

[beckyconning]

Incredibly exciting!