ENH: openfoam shell session - improved and relocated

- '-c' option (as per shell), '-Dkey[=value]' option to provide
  preferences via the command-line. For example,

      etc/openfoam -DWM_COMPILER=Clang -int64  ./Allwmake -j -s -l

  These can also be combined with other options. Eg,

      etc/openfoam -DWM_COMPILER=Clang \
          -c 'wmake -show-path-cxx -show-cxxflags'

- relocated from bin/tools/ => etc/ for easier access

- bin/tools/openfoam.in : for autoconfig-style installation

- Auto-detect if the shell script was executed with openfoam and
  interpret accordingly.

  Simple example,

      --------------
      #!/usr/bin/openfoam
      cd "${0%/*}" || exit   # Run -*-sh-*- from this dir

      blockMesh
      simpleFoam
      --------------

   Note it is NOT currently possible to provide any other parameters
   this way. Eg,

      `#!/usr/bin/openfoam -sp` (NOT)

   This will either fail to run, or result in infinite recursion.

doc/openfoam.1.in

@@ -0,0 +1,70 @@
+.TH "OPENFOAM" 1 "OpenFOAM-version" "www.openfoam.com" "OpenFOAM Commands Manual"
+
+.SH NAME
+openfoam \- OpenFOAM bash(1) session
+
+.SH SYNOPSIS
+\fBopenfoam\fR [\fIOPTIONS\fR] [\fIapplication ...\fR]
+
+.SH DESCRIPTION
+Activate an \fBOpenFOAM\fR environment in an interactive or
+non-interactive bash(1) session.
+
+If no application is given, an interactive bash session will be used.
+If an application (optionally with arguments) is provided, the
+OpenFOAM \fIetc/bashrc\fR file will be sourced from the project directory
+prior to running the application.
+
+.SH OPTIONS
+.TP
+\fB\-c\fR \fIcommand\fR
+Execute shell commands with OpenFOAM environment
+.TP
+\fB\-D\fR\fIkey=[value]\fR
+Define key/value to pass as a preference
+.TP
+\fB\-sp\fR
+Use single precision for scalar-size
+.TP
+\fB\-dp\fR
+Use double precision for scalar-size
+.TP
+\fB\-spdp\fR
+Use single precision for scalar-size, double for solve-scalar size
+.TP
+\fB\-int32\fR
+Use 32-bit label-size
+.TP
+\fB\-int64\fR
+Use 64-bit label-size
+.TP
+\fB\-prefix=DIR\fR
+Specify alternative OpenFOAM project directory
+.TP
+\fB\-show-api\fR | \fB\-version\fR
+Print META-INFO api value and exit
+.TP
+\fB\-show-patch\fR
+Print META-INFO patch value and exit
+.TP
+\fB\-show-prefix\fR
+Print project directory and exit
+.TP
+\fB\-help\fR
+Print the usage
+
+.SH ARGUMENTS
+
+If arguments remain after option processing, the first argument is
+assumed to be an application with options and arguments.
+
+.SH FILES
+
+The \fIetc/bashrc\fR file from the OpenFOAM project directory supplies
+the environment settings.
+
+.SH "SEE ALSO"
+Online documentation https://www.openfoam.com/documentation/
+
+.SH COPYRIGHT
+Copyright \(co 2020 OpenCFD Ltd.