U.S. employers posted a record number of available jobs in March, illustrating starkly the desperation of businesses trying to find new workers as the country emerges from the pandemic and the economy expands. Yet total job gains increased only modestly, according to a Labor Department report issued Tuesday. The figures follow an April jobs report last week that was far weaker than expected, largely because companies appear unable to find the workers they need, even with the unemployment rate elevated at 6.1%.