Introduce nvim namespace: Move files.

Move files from src/ to src/nvim/.
- src/nvim/ becomes the new root dir for nvim executable sources.
- src/libnvim/ is planned to become root dir of the neovim library.

src/nvim/os/shell.h

@@ -0,0 +1,45 @@
+#ifndef NEOVIM_OS_SHELL_H
+#define NEOVIM_OS_SHELL_H
+
+#include <stdbool.h>
+
+#include "types.h"
+
+// Flags for mch_call_shell() second argument
+typedef enum {
+  kShellOptFilter = 1,      ///< filtering text
+  kShellOptExpand = 2,      ///< expanding wildcards
+  kShellOptCooked = 4,      ///< set term to cooked mode
+  kShellOptDoOut = 8,       ///< redirecting output
+  kShellOptSilent = 16,     ///< don't print error returned by command
+  kShellOptRead = 32,       ///< read lines and insert into buffer
+  kShellOptWrite = 64,      ///< write lines from buffer
+  kShellOptHideMess = 128,  ///< previously a global variable from os_unix.c
+} ShellOpts;
+
+/// Builds the argument vector for running the shell configured in `sh`
+/// ('shell' option), optionally with a command that will be passed with `shcf`
+/// ('shellcmdflag').
+///
+/// @param cmd Command string. If NULL it will run an interactive shell.
+/// @param extra_shell_opt Extra argument to the shell. If NULL it is ignored
+/// @return A newly allocated argument vector. It must be freed with
+///         `shell_free_argv` when no longer needed.
+char ** shell_build_argv(char_u *cmd, char_u *extra_shell_arg);
+
+/// Releases the memory allocated by `shell_build_argv`.
+///
+/// @param argv The argument vector.
+void shell_free_argv(char **argv);
+
+/// Calls the user shell for running a command, interactive session or
+/// wildcard expansion. It uses the shell set in the `sh` option.
+///
+/// @param cmd The command to be executed. If NULL it will run an interactive
+///        shell
+/// @param opts Various options that control how the shell will work
+/// @param extra_shell_arg Extra argument to be passed to the shell
+int os_call_shell(char_u *cmd, ShellOpts opts, char_u *extra_shell_arg);
+
+#endif  // NEOVIM_OS_SHELL_H
+