* lib/pipeline-private.h (struct pipecmd): Add pre_exec_func,
pre_exec_free_func, and pre_exec_data.
* lib/pipeline.c (pipecmd_new, pipecmd_new_function,
pipecmd_new_sequencev): Initialise cmd->pre_exec_func,
cmd->pre_exec_free_func, and cmd->pre_exec_data.
(pipecmd_dup): Copy cmd->pre_exec_func, cmd->pre_exec_free_func, and
cmd->pre_exec_data if necessary.
(pipecmd_pre_exec): New function.
(pipecmd_exec): If cmd->pre_exec_func is set, call it immediately before
calling execvp or cmd->func.
* lib/pipeline.h (pipecmd_pre_exec): Add prototype.
(pipeline_install_post_fork): Cross-reference pipecmd_pre_exec in
comment.
* man/Makefile.am (FUNCTIONS): Add pipecmd_pre_exec.
* man/libpipeline.3 (Functions to build individual commands): Document
pipecmd_pre_exec.
(Functions to run pipelines and handle signals): Cross-reference
pipecmd_pre_exec from pipeline_install_post_fork.
* tests/basic.c (test_basic_pre_exec): Test pipecmd_pre_exec.
* NEWS: Document this.
* README: Update copyright years.