core: add two new unit file settings: StandardInputData= + StandardInputText=

Both permit configuring data to pass through STDIN to an invoked
process. StandardInputText= accepts a line of text (possibly with
embedded C-style escapes as well as unit specifiers), which is appended
to the buffer to pass as stdin, followed by a single newline.
StandardInputData= is similar, but accepts arbitrary base64 encoded
data, and will not resolve specifiers or C-style escapes, nor append
newlines.

This may be used to pass input/configuration data to services, directly
in-line from unit files, either in a cooked or in a more raw format.

src/core/execute.h

@@ -37,6 +37,8 @@ typedef struct ExecParameters ExecParameters;
 #include "namespace.h"
 #include "nsflags.h"
 
+#define EXEC_STDIN_DATA_MAX (64U*1024U*1024U)
+
 typedef enum ExecUtmpMode {
         EXEC_UTMP_INIT,
         EXEC_UTMP_LOGIN,
@@ -52,6 +54,7 @@ typedef enum ExecInput {
         EXEC_INPUT_TTY_FAIL,
         EXEC_INPUT_SOCKET,
         EXEC_INPUT_NAMED_FD,
+        EXEC_INPUT_DATA,
         _EXEC_INPUT_MAX,
         _EXEC_INPUT_INVALID = -1
 } ExecInput;
@@ -163,6 +166,9 @@ struct ExecContext {
         ExecOutput std_error;
         char *stdio_fdname[3];
 
+        void *stdin_data;
+        size_t stdin_data_size;
+
         nsec_t timer_slack_nsec;
 
         bool stdio_as_fds;