Make daikon.bashrc ask javac or java for JAVA_HOME

[mernst]

daikon.bashrc no longer infers JAVA_HOME only by resolving symlinks from which javac, which is wrong for wrapper scripts such as those of jenv, asdf, or mise, and for the macOS /usr/bin/javac stub. It still follows symlinks first, because that is fast. If the result is not a JDK home (it lacks a release file), daikon.bashrc asks the javac on the PATH for its java.home. It handles JDK 8, whose java.home is the jre/ subdirectory. This replaces the macOS-specific /usr/libexec/java_home call.

Daikon requires a JDK, not just a JRE, because it compiles splitters at run time. So daikon.bashrc checks that $JAVA_HOME/bin/javac is executable, rather than merely that the directory exists. The error message says whether a user-set JAVA_HOME is not a JDK or no JDK could be found.

A failed detection does not modify JAVA_HOME in the user's shell. Finding javac uses type -P, so aliases such as alias javac='javac -g' are ignored.

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[mernst]

Closing as unnecessary: #904 removes the JAVA_HOME inference from daikon.bashrc. The Makefiles compute JAVA_HOME themselves (and, with #897, java/java-home.mk does so in one place).